1. Field of the Invention
The present invention relates to a time division multiplex (TDM) switching network unit for the connection of a plurality of pulse code modulation (PCM) lines, which facilitates an assignment of time channels employed on PCM lines for the incoming direction of transmission to time channels used on PCM lines for the outgoing direction of transmission by the cyclic input of the incoming PCM words into the storage cells of an information memory, and by random reading of the outgoing PCM words from such storage cells.
2. Description of the Prior Art
Known TDM switching network units serve to establish both a spatial assignment and a time assignment between time channels on PCM lines for the incoming direction of transmission and time channels on PCM lines by the outgoing direction of transmission, and therefore they exert the function of a combined time-space stage.
From the point of view of expense, it would be most favorable to construct an entire switching network by a single switching network unit of this kind.
However, both in respect of technology and operating conditions, it is not readily possible to exceed a specific value of the number of connections which applies, in particular, when such switching network units are to be constructed in integrated circuitry.
Switching networks which are to be connected to more PCM lines than terminals exist in a switching network unit and which, accordingly, comprise a plurality of switching network units, can be designed as a single-stage or a multi-stage arrangement. In a known single-stage arrangement (Commutation & Electronique, No. 40, January 1973, Page 14-40) the switching network comprises at least two single-stage switching network components, each of which possesses as many switching network units as there are switching network components, and wherein the TDM lines for the incoming direction of transmission which are connected to the switching network units are also connected to one input of the switching network unit of each other switching network component. When the switching network is designed in this manner, however, the increase in the number of connections leads to a quadratic increase in the number of switching network units which are used.
At least from the point of view of the expense, in terms of requisite switching network units, the aforementioned multi-stage switching networks are more favorable. With respect to blocking-free operation, it is sufficent, in multi-stage switching networks of this kind, that time channel conversions should be possible in the input end stage and in the output end stage. On the other hand, the intermediate stages can represent pure space-switching stages. Consequently, the construction of a multi-stage switching network purely from identical switching network units which, as stated above, can result fully both in a spatial assignment of time channels and in a time-wise assignment of time channels represents an unnecessary expense.
Therefore, on the one hand, there is an interest in switching network units which represent pure spatial switching stages, but which, from the technological viewpoint, largely conform with the aforementioned time-space stages which operate on the basis of time shifts.
Multi-stage switching networks constructed employing switching network units are more favorable, from the stand-point of expense in switching network units, than the aforementioned single-stage switching networks, although they result in longer switch-through transit times and in a larger expense in the setting-up control system.
Therefore, interest also exists in spatial switching units of the above-mentioned type, wherein it is possible to connect a larger number of PCM lines than was previously the case for the above-given reasons, so that even when a multi-stage construction is needed it is at least possible to keep the number of stages low.